Why port Emacs to Rust ?, it adds no value to users. Why not develop a modern GUI like Intellij IDEA with a Common Lisp implementation underneath like SBCL ?
Any person who write their own license to specify in fine details how the work may be used and distributed is going to have a very strong (and likely legal) issue with people who try to circumvent that license. Even a clean room implementation would need source material as instructions, and writing such specification without any copyrighted elements of the source would be quite a challenge when dealing with a very large program that has compatibility to a large ecosystem as its primary feature.
Apple chose FreeBSD as base for it OS, Google chose Linux. Look at sad state of FreeBSD now.